A power supply apparatus, e.g. a power supply apparatus for a welder, may have a control panel on its outer surface. An example of a power supply apparatus with such control panel is disclosed in Japanese Patent Application Publication No. 2003-136241 (JP 2003-136241 A). The power supply apparatus disclosed in JP 2003-136241 A includes a control panel on an outer surface of a case, and the control panel includes setting devices for setting currents and voltages the power supply apparatus may supply, switches for selecting operating modes of the apparatus, and indicator lamps indicating the operating states. Also not explicitly stated in the publication, the setting devices, the switches and the lamps are mounted on a printed circuit board. This printed circuit board may be packed with high density. In such case, the distance between printed conductors on the circuit board is very small, e.g. less than 0.5 mm. The densely packed printed circuit board is fixed to the rear, or inside surface of the control panel. Plural openings are formed through the control panel, through which controls for the setting devices and switches and the indicator lamps are exposed to enable an operator to externally operate the setting devices and switches and to confirm the operating states by means of the indications by the lamps.
Frequently, in order to cool the interior of the case of the power supply apparatus, outside air is taken into the case, and, in such case, both of opposing major surfaces of the densely packed printed circuit board for the control panel is exposed to the air taken into the case.
A power supply apparatus for a welder is sometimes used in an environment where electrically conductive dust and/or corrosive gas are present. Such conductive dust and corrosive gas will enter into the case of the power supply apparatus and adversely affect printed circuit boards inside the case. In particular, the printed circuit board for the control panel is affected by dust and corrosive gas more than other printed circuit boards, because it is more densely packed. Electrically conductive dust sticking to the printed circuit board degrades insulation of the circuit board, and corrosive gas corrodes components mounted on the circuit board and printed conductors, which would cause malfunction of the power supply apparatus.
Therefore, an object of the present invention is to protect a printed circuit board for a control panel from surrounding air to avoid influence of the air on the printed circuit board.